1. Field of the Invention
The present invention is directed to tissue engineering and, more particularly, to creating a three dimensional construct of cells and mechanically conditioning the construct.
2. Description of Related Art
Connective tissue cells from muscle, bone, tendon, ligament, and cartilage respond to mechanical loading. Many types of devices have been developed to apply static or cyclic strain to cells. These devices include reciprocating arms clamped to a matrix substrate, weights placed upon cells grown on a distensible membrane, and a forcing frame in which cells on a distensible substrate are statically stretched.
A particular device, conceived by the present inventor, applies static or cyclic tension or compression to cultured cells grown on a deformable substrate. The deformation of the substrate is regulated by pressure controlled by a solenoid valve and a timer. In one embodiment of the device, a vacuum is used to downwardly deform a polystyrene surface on which tendon cells are attached. The cells respond by altering their synthesis of cytoskeletal proteins. This device evolved into a computer controlled device that provides regimens of strain having defined duration, frequency, and amplitude which are either static or cyclic. A culture plate that allows easy growth of cells on a flexible bottom culture plate is used with this device. This device is known as a Flexercell® Strain Unit (FSU) provided by Flexcell International Corporation. The FSU controls devices that provide regulated strain in the form of tension, compression, and fluid-induced shear stress to cells. These types of strain encompass the broad areas of strain types experienced by a multitude of cells in native environments of the body. This device created a new field of study known as cytomechanics and has provided a standard instrument and culture plate for broad use in the marketplace. For reference, see U.S. Pat. Nos. 6,218,178; 6,048,721; and 5,518,909.
Research by Flexcell International Corporation shows that cells that have been subjected to a particular regimen of cyclic strain followed by rest (for example, 1 Hz, 1% substrate elongation, 8 hours/day for 3 days) respond with an increased ability to attach and spread on a substrate and resist removal by fluid flow. Flexcell International Corporation calls this cell-based training response “mechanical conditioning”. It is theorized that different training regimen will produce different training responses in cells, including the ability of cells to attach, spread, and adhere to a matrix; organize an oriented three dimensional matrix; express a native matrix; maintain a level of cell division; and in general, maintain a native phenotype that is biochemically and biomechanically normal.
Cells in tissue environments in the body are present in three dimensional matrices. These matrices have their own particular anatomy, material structure, and biomechanical properties. As a tissue develops, its cells require an appropriate degree of mechanical deformation, as well as nutrition supplied by either diffusion from extracellular fluids or by delivery of nutrients through the blood vascular system. In order to develop and utilize tissue-engineered, biocompatible materials that can sustain cell growth in vitro and withstand the rigors of the biomechanical environment, a proper structure design as well as means to provide nutrient availability must be created. The structure must: 1) support cell attachment and growth; 2) be easy to infiltrate with cells; 3) be biocompatible, that is of low immunogenicity or antigenicity if it is to be implanted in an animal or a human; 4) have a structure compatible with cell division followed by differentiation, matrix expression, and differentiated function; 5) have a structure that allows collection of a bioproduct and/or cells; 6) be chemically stable; 7) be easily obtained, formed, manufactured, and sterilized; and 8) be inexpensive.